CanCan::Ability#merge transferring only rules

main.rb

begin
  require 'bundler/inline'
rescue LoadError => e
  $stderr.puts 'Bundler version 1.10 or later is required. Please update your Bundler'
  raise e
end

gemfile(true) do
  source 'https://rubygems.org'
  gem 'rails', '5.1.0'  # use correct rails version
  gem 'cancancan' # use correct cancancan version
  gem 'sqlite3' # use another DB if necessary
end

require 'active_record'
require 'cancancan'
require 'cancan/model_adapters/active_record_adapter'
require 'cancan/model_adapters/active_record_4_adapter'
require 'minitest/autorun'
require 'logger'

# This connection will do for database-independent bug reports.
ActiveRecord::Base.establish_connection(adapter: 'sqlite3', database: ':memory:')
ActiveRecord::Base.logger = Logger.new(STDOUT)

# create your tables here
ActiveRecord::Schema.define do
  create_table :books, force: true  do |t|
    t.integer :user_id
  end

  create_table :users, force: true  do |t|
    t.string :name
    t.string :role
  end
end

class Book < ActiveRecord::Base
  belongs_to :user
end

class User < ActiveRecord::Base

  ROLES = {
    user: 'user',
    admin: 'admin'
  }.freeze

  has_many :books

  def role_in?(roles)
    roles.map { |role| self.class::ROLES[role] }.include? role
  end

end

class AbilityBuilder

  attr_reader :ability, :user

  def initialize(ability)
    @ability = ability
    @user = ability.user
  end

  def apply(other)
    target = build_target.extend(other)
    target.abilities(user)
    ability.merge(target)
    self
  end

  def build
    ability
  end

  private

  def build_target
    target = Object.new
    target.extend(CanCan::Ability)
  end

end

module RoleAbility

  def authorized?(user)
    user.role_in?(roles)
  end

  def abilities(_user)
    raise NotImplementedError
  end

  def roles
    raise NotImplementedError
  end

end

module AdminRoleAbility

  include RoleAbility

  def abilities(user)
    manage_books if authorized?(user)
  end

  def roles
    %i[admin]
  end

  private

  def manage_books
    alias_action :create, :read, :update, :destroy, to: :crud
    can :crud, Book
  end

end

class Ability

  KNOWN_ABILITIES = [AdminRoleAbility].freeze

  include CanCan::Ability

  attr_reader :user

  def initialize(user)
    @user = user
    builder = AbilityBuilder.new(self)
    KNOWN_ABILITIES.reduce(builder) { |memo, ability| memo.apply(ability) }
  end

end

class BugTest < Minitest::Test

  def test_bug
    user = User.create!(role: 'admin')
    Book.create([{user: user}])
    ability = Ability.new(user)
    books = Book.accessible_by(ability, :destroy).count
    assert_equal 1, books
  end

end